In the case of one fuel injector (German document DE 101 08 945 A1), the valve needle is connected with force locking to a magnet armature of an electromagnet and carries a closing member which, together with the valve seat, forms a sealing seat on the valve opening, which is produced for closing the valve and canceled out for opening the valve. To produce the sealing seat, a return spring, which is supported on the end of the valve needle facing away from the closing member and on an adjusting sleeve, presses the closing member onto the valve seat. The valve needle has a collar-shaped armature stop and a driving flange which make possible a so-called prestroke of the magnet armature, by which the magnet armature is able to be displaced axially on the valve needle. A prestroke spring configured as a helical compression spring, in this case, presses the magnet armature against the armature stop with force locking. When a current is applied to the electromagnet, the armature is moved, against the spring force of the prestroke spring and the return spring, in the lift of stroke direction. In this context, the overall lift of stroke is subdivided into the prestroke and the actual opening lift of stroke having the lifting off of the closing member from the valve seat. During the prestroke, the closing member still remains on the valve seat. The opening lift of stroke sets in as soon as the armature strikes the driving flange after the end of the prestroke, and via the latter, drives the valve needle in the lift of stroke direction.
Such a valve has the advantage over a valve, having a rigid connection of armature and valve needle, that the inert mass moved in response to the valve closing is divided up into two partial masses, namely into the magnet armature and the valve needle having the closing member, which leads to a reduction in noise during the closing of the valve. The prestroke spring and the armature free play, on the one hand, improve the opening dynamics of the valve and, on the other hand, damp so-called bounce pulses, which lead to a metering quantity of fluid that is not reproducible. The opening dynamics are achieved by the so-called mechanical boostering, in that the prestroke spring that is prestressed in the closed state of the valve accelerates the magnet armature in addition to the magnetic force of the electromagnet, so that the valve may be opened using a lesser force expenditure. The bounce pulses are avoided by the damping effect of the prestroke spring, since the prestroke spring, during the closing of the valve after the first hitting of the closing member on the valve seat, damps a motion of the magnet armature in the lift stroke direction created by the rebounding of the magnet armature, and thus prevents a renewed, brief lifting off of the closing member from the valve seat made possible by the magnet armature striking the armature stop.
Additional valves having a prestroke spring and an armature free play are discussed in DE 198 49 210 A1, DE 199 32 763 (FIG. 2) and DE 199 46 602 A1.